The invention relates to a flow-meter for measuring the rates of flow of fluids, more particularly adapted to the requirements of measurement in the field of automobiles, especially for fuel measurement.
Characteristics of the automobile field include the very wide range of operating temperatures (-40.degree. C. to +125.degree. C.), the stresses of vibrations and impacts, the presence of corrosive agents. Finally and primarily, the essential characteristic is the necessity of very low cost whilst maintaining good performances.
The list of specifications of an automobile fuel flowmeter is roughly as follows:
measurement range: from 0.5 to 50 liters per hour, to avoid duplication of ranges; PA1 compensated operating temperature: -30.degree. C. to +80.degree. C.; operating temperature: -40.degree. C. to +125.degree. C.; PA1 impacts and vibrations: impacts 50 g., vibrations 3 g..+-.10 mm. from 5 to 50 Hz in the three dimensions; PA1 loss of pressure less than 40 mbars; PA1 indifference to bubbles; PA1 indifference to pulsating flow with reversal of the direction of circulation; PA1 density of the fuel: 0.745.+-.3%, with a temperature coefficient of 8.5.multidot.10.sup.-4 per .degree.C.; PA1 instantaneous measurement precision better than 10%; PA1 cumulative measurement precision, taking account of all variations, better than 5%; PA1 small bulk and ease of fitting; PA1 analog or numerical (frequency) output signal with a preference for numerical; PA1 reduced electrical connections.
Let it be noted that this list of specifications opens up many applications other than the field of automobiles.
As regards indication, analog indication is useful for operational learning of economical driving, while numerical indication permits quantitatively monitoring the consumption, which then requires maximum possible precision.
At present, different techniques seek to respond to all these requirements. First there are axial turbine flowmeters, which have the drawback of being sensitive to bubbles and of having internal frictions which render their precision poor at low flow rates (below 3 l./h.). Moreover the fact that it is not possible to standardize them or effect compensation simply necessitates precise and costly manufacture. They possess the advantages of simplicity of their electronics and the directly numerical character of their read-out.
A flow-meter type with a tangential turbine currently has characteristics which make it a very valid apparatus. However, despite the fact that at low flow rates its operation is better than that of preceding apparatuses, it also possesses the same drawbacks.
A flow-meter close to the invention here described in principle is a flow-meter having a pressure loss which is not zero at zero flow rate, then increases as a function of the flow rate. A cone is displaced, under the effect of the pressure generated by the flow rate, in another, fixed, cone of smaller angle. A spring tends to apply the first cone to the second. A rod fast with the first permits measurement of the displacement by a differential transformer. In the case of this detector only the force exerted upon the first cone by the pressure due to the pressure loss at the level of the throttling action between the cones displaces the first cone. In view of the necessary small section of the first cone, thus the force is very slight and subject to interference by numerous parameters (friction, inclination, etc.). Moreover the throttling effect between the cones is such that the pressure losses by friction are not zero and that they greatly disturb the measurement at low flow rates. Moreover these pressure losses depend upon the viscosity of the fluid, the value of which moves by about 10% per 10.degree. C., which causes a large temperature drift. Finally this technique necessitates an almost perfect centering of the fixed part in relation to the movable part.
A different flow-meter type is a differential pressure pick-up flow-meter utilising a contactless displacement pick-up. The drawback of this apparatus, apart from its low dynamics (0.5 to 25. l./h.), is the imperfect stability of its zero, due to the fact that the pressure loss is approximately quadratic and that the gain must be very large at low flow rates, rendering the apparatus sensitive to several stray parameters such as swelling of the diaphragm, inclination of the apparatus, hysteresis of the springs.
Its advantages are its low cost price, the fact that it has low sensitivity to bubbles, the fact that in itself it constitutes a fluid filter permitting excellent operation in pulsating or impulse rate, that it is equipped with an analog transducer which permits standardisation and thus a widening of the mechanical tolerances, and that it can be temperature-compensated.